ES2964492T3 - electric arc furnace - Google Patents

Abstract

A direct current plasma arc furnace (1) for melting and/or treating a material by producing electric arcs and providing a molten product, comprising the furnace (1); a tank (10) comprising: a crucible (12) that delimits a chamber (11) configured to receive material to be melted and/or treated; a plurality of refractory walls surrounding the outer surface of the crucible (12); and a metal frame (15) that covers the refractory walls; and a heating system configured to heat the received material, the heating system comprising a first electrode that acts as a cathode (13) and a second electrode (120) that acts as an anode, where the first electrode that acts as a cathode (13 ) is a mobile electrode configured to project vertically into the chamber (11) delimited by the crucible (12). The crucible (12) is made of a material that comprises at least 98% synthetic graphite, the crucible (12) being part of an anode system that also comprises said second electrode (120) and at least a part (12' , 12") connect the crucible (12) and the second electrode (120), said crucible (12) having a double function: receiving and retaining material to be melted and/or treated and providing electrical conduction for the flow of current to heat said material, in such a way that, in use of the furnace (1), the voltage potential difference between the cathode (13) and any point on the surface of the crucible (12) defined to be in contact with the material to be melted and /o treaty is the same. (Automatic translation with Google Translate, without legal value)

Description

DESCRIPTION

electric arc furnace

Technical field

The present invention relates to the field of electric arc furnaces for chemical and/or metallurgical applications, such as the melting and/or treatment of metals and/or metal waste and/or byproducts containing metals and other chemical compounds, such as metal oxides.

State of the art

A wide variety of furnaces whose geometry, procedure and heating systems differ significantly and are used in different chemical and metallurgical applications. Depending on their mode of operation, furnaces can be grouped into continuous or batch furnaces, which can use electricity or fossil fuels. They can also be classified according to their geometry. They can be of direct or indirect application. The advantages of each type of furnace depend on several factors, such as the type and size of the charge used, since the charge determines the energy efficiency and the metallurgical quality resulting from the applied process, such as the melting or treatment process.

A well-known type of furnace that uses electricity is the direct current (DC) electric arc furnace. An example of a DC electric arc furnace is disclosed in US4466824. The furnace has a crucible to collect the molten material. The surface of the crucible designed to be in contact with the molten material is made of refractory material. An anode is mounted in the lower section of the crucible. A cover is arranged in the upper part of the oven. A cathode is inserted into the furnace through a central opening made in the cover. During furnace operation, when the tip of the cathode is very close to the molten material filling the crucible, a plasma arc is initiated. The patent document KR 101 731 990 B1 has the specificity that the anode is mounted on a rod support placed below the direct current electric arc furnace.

In other DC arc furnaces, the anode is built into the crucible in the form of different types of electrical conductors inserted into the refractory material from which the crucible is made. This is disclosed, for example, in US4541099, US5381440 and US5381441.

However, in the cited disclosures there are local electrical connection areas that may have poor conductivity when starting operation with low temperature loads (e.g. <750°C for ferrous loads or <450°C for non-ferrous copper loads). or even at room temperature from the cold start of the oven. Poor conductivity in the electrical circuit can generate enormous energy losses and even malfunction of the installation. Likewise, these types of local connections can completely lose electrical conductivity due to cavities that form in the charge during melting or furnace charging.

Description of the invention

The present invention provides an improved electric arc furnace for chemical and/or metallurgical applications, such as melting and/or treating metals and/or metal scraps and/or by-products containing metals and other chemical compounds, such as metal oxides. , whether simple or complex metal oxides.

The oven of the invention comprises a chamber delimited by a crucible. The chamber has the double function of receiving the material to be treated and heating it for treatment. The furnace is heated by an electrical system, such as a plasma torch electrical system, which has two electrodes: cathode and anode. The electrical system is equipped with a DC power supply. During furnace operation, the electrodes allow current to flow from one electrode to the other. The anode of the plasma system is the surface that delimits the chamber, that is, the crucible of the furnace. The cathode is an electrode, preferably a graphite electrode, arranged at the top of the chamber. The cathode can move vertically, towards the chamber containing the material to be heated. The cathode and anode are electrically isolated from each other. During operation, when the tip of the cathode is very close to the molten material filling the crucible, that is, the anode, a plasma arc is initiated. The main body of the furnace is made of one or more refractory materials surrounding the crucible.

According to a first aspect of the present invention, a direct current plasma arc furnace is provided for melting and/or treating a material by producing electric arcs and providing a molten product. The furnace comprises: a tank comprising a crucible that delimits a chamber configured to receive material to be melted and/or treated, a plurality of refractory walls that surround the outer surface of the crucible and a metal frame that covers the refractory walls; and a heating system configured to heat the received material, the heating system comprising a first electrode that acts as a cathode and a second electrode that acts as an anode, wherein the first electrode that acts as a cathode is a mobile electrode configured to project vertically inside the chamber delimited by the crucible. The crucible is made of a material comprising at least 98% synthetic graphite, the crucible being part of an anode system that also comprises said second electrode and at least a part that connects the crucible and the second electrode, said crucible having a dual function: receive and contain material to be melted and/or treated and provide electrical conduction for the flow of current to heat said material, in such a way that, when the furnace is in use, the voltage potential difference between the cathode and any point on the surface of the crucible defined to be in contact with the material to be melted and/or treated is the same. At least a portion of the anode system comprises: a first elongated portion having a first end connected to the bottom wall of the crucible and extending radially therefrom; and a second elongated portion having a first end connected to the second end of the first elongated portion, the second elongated portion extending vertically until its second end protrudes out of the oven.

In embodiments of the invention, the heating system comprises a plasma torch.

In embodiments of the invention, the cathode is made of a material comprising at least 98% by weight of synthetic graphite.

In embodiments of the invention, the plurality of refractory walls arranged between the outer surface of the crucible and the metal frame comprises: a first layer of refractory material that vertically surrounds the perimeter wall of the crucible; a second layer of refractory material vertically surrounding the first layer; a third layer of refractory material arranged under the bottom of the crucible; and a fourth layer of refractory material that vertically surrounds the second layer and in contact with the outer metal frame.

The first layer preferably comprises alumina-based corundum. More preferably, it is made of self-sintering alumina-based corundum having an amount of alumina greater than 85% by weight.

The second layer preferably comprises alumina-based concrete. More preferably, it is made of alumina-based formed concrete having an amount of alumina greater than 90% by weight.

The third layer and the fourth layer preferably comprise calcined bauxite. More preferably, they also comprise alumina-based concrete.

In embodiments of the invention, the fourth layer extends downward with respect to the third layer, surrounding the previous refractory portions disposed around the crucible.

In embodiments of the invention, the furnace further comprises a fifth layer of refractory material surrounding the second elongated portion of the anode system, the fifth layer being a self-sintering silica-based refractory.

In another aspect of the invention, the use of the oven described above is provided for melting or treating metals and/or metal scraps and/or by-products containing metals and other chemical compounds.

Thanks to its design and geometry, the proposed furnace is highly efficient and has low energy consumption and high metallurgical performance. Additionally, the furnace is versatile as it can be used to melt/treat different types of materials.

Additional advantages and features of the invention will become apparent from the following detailed description and will be especially noted in the appended claims.

Brief description of the drawings

To complete the description and in order to provide a better understanding of the invention, a set of drawings is provided. Said drawings form an integral part of the description and illustrate an embodiment of the invention, which should not be construed as restricting the scope of the invention, but only as an example of how the invention can be carried out. The drawings include the following figures:

Figures 1A-1C show different views of an oven according to an embodiment of the invention.

Figure 2 schematically shows a cross-sectional view of a tank comprised in the oven of Figure 1, in accordance with an embodiment of the invention.

Figure 3 schematically shows a cross-sectional view of an oven according to an embodiment of the invention.

Figures 4A-4D show different views of the parts that form the anode of the heating system of the oven of Figure 1.

Figures 5A-5B schematically show the parts that form the anode, once assembled, of the oven of Figure 1.

Figure 6 schematically shows an electrical circuit for powering the oven, according to one embodiment of the invention.

Figure 7 shows a side view of the oven according to an embodiment of the invention, in which an extraction duct for extracting fumes from the oven is illustrated.

Description of a way of carrying out the invention

The following description should not be taken in a limiting sense, but is provided solely for the purpose of describing the general principles of the invention. The following embodiments of the invention will be described by way of example, with reference to the above-mentioned drawings showing apparatus and results according to the invention.

With reference to the figures, a possible embodiment of the oven of this invention is described below. The furnace is a melting furnace, meaning it processes material in a molten state. The resulting product after the treatment applied in the oven is in a liquid state. The oven of the invention is based on the direct heating of the charged material by means of a direct current (DC) plasma arc, also known as a DC plasma jet, obtained from two electrodes. In particular, the oven 1 is heated by a heating system, such as a plasma torch system, comprising two electrodes: a first electrode acting as a cathode 13 (negative electrode) and a second electrode acting as an anode 120 ( positive electrode), as shown, for example, in Figures 1A-1C. Reference 120 denotes the anode connection. As can be deduced from the side view shown in Figure 1A, the CC arc furnace 1 comprises a tank, container or vessel 10 defining a chamber or interior volume 11 (seen for example in the cross-sectional view of the tank of Figure 2). Preferably, the chamber 11 is a substantially central chamber in the furnace 1. During use of the furnace 1, the chamber 11 is filled with material - such as melt - to be treated, such as metals, metal scraps or by-products containing metals and other chemical compounds, such as metal oxides. The interior walls (bottom and side walls) of the oven that define the chamber 11 constitute a crucible 12. In other words, the crucible 12 receives the material to be treated, where it is heated. Therefore, the crucible 12 has the double function of receiving the material to be treated and heating/treating the material. As will be described in detail with reference to Figure 2, the crucible 12 is surrounded by one or more layers or walls or refractory material. Lastly, the outer layer of oven 1 is made of metal. In other words, a metal frame 15 surrounds the outer refractory walls, forming a metal container. The metal frame 15 allows the thermal expansion of the materials that form the refractory layers, preventing the formation of cracks in said materials. A correct design in terms of symmetry, well-established distances and thermal insulation allows for homogeneous heating and, therefore, an optimized oven.

The metal frame 15 is also suitable for supporting the oven 1 in order to tilt it for pouring the melt when the batch/pouring is ready. Crucible 12 and its surrounding layers (refractory layers and metal structure) are not cooled. In other words, there is no cooling medium surrounding the crucible. This is because furnace 1 is intended to melt and treat material, which never solidifies inside crucible 12. On the contrary, once melted/treated, the liquid material (molten metal) is removed from the furnace. For this reason, the use of cooling media surrounding crucible 12 is discouraged. Furnace 1 is therefore not suitable for producing solid blocks within crucible 12.

In the embodiment shown in Figure 1A, the oven 1 is mounted on a platform 20. The platform 20 allows the oven 1 to be tilted, for example for cleaning purposes. Therefore, furnace 1 can be tilted up to 90° for metal extraction and partial or complete emptying (porification/casting). During this operation, the slag can also be removed from the furnace and the crucible cleaned. If, during operation, partial slag removal is necessary, depending, for example, on the characteristics of the molten material, this may not be done correctly and safely using conventional tilting systems. For this reason, platform 20 is used, which allows the oven to be slightly tilted perpendicular to the appropriate tilt direction. This allows for slag cleaning while the molten/refined metal remains in the crucible. Alternative ways of extracting the molten metal from the furnace 1 may be manual extraction or extraction using pumping means, among others.

A part of the load (material being treated/heated) can be gasified, in which case it is collected with a dust filter system, which has a smoke capture medium in the upper part of the oven, around the upper ring. . The smoke is captured in the upper area of the furnace, conducted and extracted from the furnace through the extraction duct 25 (see Figure 7), from which the smoke is supplied to a main smoke capture system, not shown, which is outside the scope of the present invention.

The oven 1 also comprises a cover 21, as shown, for example, in Figure 1B, which closes the tank 10 during operation of the oven for safety reasons. The cover 21 is opened or closed by actuation means 23, which is outside the scope of the present invention. The cover 21 has a hole, inlet or through hole 26 through which the electrode acting as cathode 13 is inserted into the tank 10, that is, in the volume 11 defined by the crucible 12. The hole 26 is preferably located in the center of the cover 21, the cover 21 being preferably centered with respect to the chamber 11. The gap 26 can have a maximum displacement of approximately 15% (measured with respect to the diameter of the chamber) with respect to the center of the cover 21. The material to be fed to the crucible 12 for melting is transported to the crucible 12 through a second hole, inlet or through hole 27 in the cover 21. Material feeding can be carried out, for example, by a system vibratory feeding. Material can be loaded manually, with metering screws, or by any other suitable loading means.

During operation, the furnace, in particular the crucible 12 and the material contained therein, is heated by a heating system comprising the two electrodes, cathode and anode, necessary to allow current to flow from one electrode to the other when the DC power supply with which the heating system is equipped is turned on. Figure 6 illustrates a block diagram of the electrical circuit for powering the furnace 1. The cathode connection 13 is electrically connected to a corresponding connection on an electrical panel or plate 51. Similarly, the anode connection 120 is electrically connected to a corresponding connection on the electrical panel 51. Both connections are typically made with a conventional DC electrical cable 28. These cables 28 are preferably cooled by cooling media, such as water. The electrical panel 51 is in turn electrically connected to a rectifier 52. The connections of the cathode 13 and the anode 120 are also shown in the cross-sectional view of Figure 3.

The cathode 13 is a moving electrode. In embodiments of the invention it is the moving part of a plasma torch. Due to its linear movement, it provides the oven with the ability to regulate power according to the operating conditions of the oven. The electrode that acts as cathode 13 is introduced - thanks to its linear movement - through the hole 26 inside the tank 10, that is, into the volume 11 defined by the crucible 12. Thanks to the linear movement of the plasma torch cathodic, the power at the chamber input can be adjusted, since the position of the moving electrode can be changed depending on the electrical resistivity of the load (fusion, etc.). Through the hole 26 made in the cover 21, the electrode 13 protrudes in a vertically movable manner in the volume defined by the crucible 12 and forms the cathode 13 for the DC electric arc. Therefore, cathode 13 is capable of linear movement.

The electrode 13 is held, for example, by a movable support arm 22 actuated with an electrically active connector 24, such as a clamp. The clamp has both mechanical and electrical functions (holds the support arm 22 and performs electrical wiring). The connectors 24 (such as clamps) also connect the electrodes - the cathode 13 and the anode (crucible) 12 - with the required electrical wiring 28. The electrical wiring 28 is schematically represented in Figure 6. The clamp 24 is made, for example, of a copper alloy. In other words, it may be made of copper-based compounds, for example compounds comprising at least 75% copper and optionally at least one of Zn/Sn/Ni/Al/Fe. The clamps 24 are designed to facilitate disassembly and replacement of electrodes. The clamp 24 is cooled, for example by water. This assembly - movable support arm 22 and electrically active clamp 24 - is made by means of a coating insulation part that has high electrical resistivity and thermal resistance, thus allowing working temperatures greater than 200 °C. Insulation means and cooling means are outside the scope of the present invention.

Figure 6 shows the main parts or elements of the plasma torch electrical circuit, including the electrical wiring that connects to the plasma torch and the plasma torch system itself. An electrical distribution cabinet 52, also known as a rectifier, acts as a direct current (DC) source. From the electrical distribution cabinet 52, power is directed to an electrical panel or plate 51, for example using a rigid cable or rigid wiring. The electrical panel or panel 51 is a connection exchanger that receives a rigid cable or wiring as input and supplies a refrigerated cable (refrigerated wiring) 28 at its exit. In other words, the energy exchanger 51 allows cooling the electrical connection to the electrodes 13, 120 and therefore towards the plasma torch. The energy exchanger 51 generates a difference in voltage value between the electrodes 13, 120, which produces the ionization of a gas (air, nitrogen or argon) between electrodes and thus allows the passage of electric current from one electrode to the other. The ionization process begins with the emission of electrons from the cathode to the anode, which is actually the electric current. The parts of the oven 1 that are cooled (refrigerated) are preferably the DC electrical cables 28 and the assembly formed by the mobile support arm 22 and the electrically active connector 24. The body of the oven (crucible and surrounding walls) and auxiliary elements , except the wiring and clamping electrodes, are not water cooled.

Cathode 13 is made of a carbon-based inorganic material. The cathode 13 is preferably made of synthetic graphite with a purity equal to or greater than 98%. In other words, the cathode 13 is preferably made of a material comprising at least 98% synthetic graphite (percentage expressed by weight with respect to the total weight of the material) and an amount of ash content equal to or less than 2% ( percentage expressed by weight with respect to the total weight of the material). As an example, the material from which the cathode is made may be produced using a feedstock that has at least 30% acicular coke.

As shown in Figure 2, the electrode that acts as the anode is the crucible 12. In other words, the crucible 12 (part of the furnace that defines the chamber or interior volume 11 defined by the tank 10) constitutes the anode. The anode (crucible 12) therefore has a double function: as a crucible, it receives and holds the material to be treated (such as metal, waste or molten material); As an anode, it provides electrical conduction for current flow in the heating system. The heating system provides the chamber 11 defined by the crucible 12 with sufficient energy to melt and treat the materials with which the chamber 11 has been filled. The bottom and side walls that define the inner chamber 11 are therefore the anode. 12 of the heating system, as shown in Figure 2. The cathode 13 is isolated, from an electrical point of view, from the anode 12. During operation, the anode 12 is well protected by the melt filling the crucible, by the impact of the plasma arc. The layer 12 that forms the crucible, and also the anode of the heating system, is made of a carbon-based inorganic material. The layer 12 is preferably made of synthetic graphite having a purity equal to or greater than 98%. In other words, it is preferably made of a material comprising at least 98% synthetic graphite (percentage expressed by weight with respect to the total weight of the material) and an amount of ash content equal to or less than 2% (percentage expressed by weight with respect to the total weight of the material). As an example, the material from which the cathode is made may be produced using a feedstock that has at least 30% acicular coke. Compounds comprising at least 98% carbon have high electrical and thermal conductivity. Therefore, both the cathode and the crucible, that is, the anode, have high electrical and thermal conductivity.

Figure 2 shows a possible implementation of the heating system, in particular the plasma torch heating system. The anode system (crucible-anode system) is formed by the crucible 12 itself and at least a part connecting the crucible 12 and the second electrode 120. To facilitate construction and maintenance, the anodic system (crucible-anode system) can be formed by three parts: the crucible 12 itself (perimeter wall and lower wall that define chamber 11); a first elongated portion 12' having a first end connected to the bottom wall of the crucible 12 and extending radially from the bottom wall of the crucible through a refractory wall (not yet disclosed in detail); and a second elongated portion 12'' having a first end connected to the second end of the first elongated portion 12'. The second elongated portion 12'' extends vertically through a refractory wall or between two refractory walls (not yet disclosed in detail) until it appears at the upper end of the oven 1, in the vicinity of the cover 21. In the Figure 2, the free end of the second elongated portion 12'' is designated 120 (anode connection), in line with the corresponding element 120 in Figure 1. Preferably, the three parts 12, 12', 12'' are made substantially with the same material. They are made of an inorganic carbon-based material. Preferably they are made of synthetic graphite with a purity equal to or greater than 98%. In other words, they are preferably made with a material comprising at least 98% synthetic graphite (percentage expressed by weight with respect to the total weight of the material) and an amount of ash content equal to or less than 2% (percentage expressed by weight with respect to the total weight of the material). As an example, the material from which they are made may be produced using a raw material that has at least 30% acicular coke. The difference in conductivity in the three parts 12, 12', 12" is preferably less than 5%, or less than 4% or less than 3%.

The inventors have observed that having an anode corresponding to the crucible 12 provides the effect of having the same voltage potential difference across the entire outer surface of the crucible. This makes the heating system more stable than conventional ones, in which the anode is implemented, for example, as a group of electrical plates arranged in a certain portion of the crucible. As a consequence, the temperature of the material to be treated inside the crucible is more stable. In other words, the material (metal, waste, etc.) is treated at the anode itself. Figure 4A shows a view of the crucible 12 and the cavity 11 thus defined, according to a possible implementation of the oven. The shown geometry of the anode allows better control of the plasma jet due to the isopotential (isoelectric potential or isovoltage potential) at all points on the walls (floor and side walls) of the crucible. During the operation of the furnace 1, the material maintained in the crucible 12 at a high temperature, such as to melt, is between the anode and the cathode, and in direct contact with the anode, as a result of which it reaches the same voltage as the anode (crucible 12) and actually acts as an anodic part in the plasma system. In other words, the larger the surface area of the anode (crucible 12), the easier it will be for the electrons emitted by the cathode to reach the anode. This is especially important when the jet starts (at the beginning of the operation) or in case of poor conductivity conditions due, for example, to the type of material being loaded into oven 1.

At the beginning of the furnace operation, that is, at room temperature, the material to be loaded is preferably metallic, or at least has an electrical conductivity similar to that of metallic materials, since a minimum electrical conductivity is required for start-up. until a certain temperature is reached (for example, approximately 800 °C). In this way, the charged material also acts as an anode in the plasma torch system. For example, an amount of metal material is required that fills the crucible to a depth of approximately 10 mm. The material is then melted under the heating action of the energy radiated by the plasma column 19 and maintained in a liquid state in the crucible 12. Once the initial conditions have been met and a certain temperature has been reached, the furnace can be loaded with material. non-metallic material (generally with material with poor conductivity) to melt it, since at the operating temperature non-metallic materials are sufficiently conductive to achieve substantially the same voltage as the anode. Non-conductive materials are melted by the effect of the plasma jet through heat transfer by radiation and convection (up to temperatures exceeding 1,000 °C, for example). The electrons generated at the cathode 13 flow into the plasma column 19 and are collected on the surface of the molten pool 17 which acts as the anode surface, thus releasing their recombination heat and heating the molten pool 17. It is estimated that the maximum level of molten metal is, for example, approximately 2/3 of the height of the chamber (Figure 3).

In summary, the anode provides high conduction to current flow and stabilizes the plasma jet within the cavity 11, because the crucible 12 provides an isoelectric potential surface. In this text, when the crucible is said to be isopotential, an error or tolerance of approximately /-1% must be assumed. In particular, the disturbances in the output voltage are less than 5% of a configuration target. The setup target consists of voltage and current targets selected to obtain the desired output power. The voltage and intensity values are parameters of the heating process and are related to the electrical conditions between the anode and the cathode.

At the start of the operation, the charge (material) to be processed can be added in solid form (normally at room temperature) and then melted in the furnace. Alternatively, it can be transferred directly in liquid form from another oven. Normally, the material to be treated is loaded in a solid state, starting operation at room temperature and heating the oven as explained, up to the appropriate operating temperature, which depends on the process being carried out. The operating temperature is maintained for the time necessary to process the material. The material - added, for example, in a solid state - remains in contact with the anode (because the entire crucible is an anode, it is an isotension zone), as a consequence of which all the material fed is substantially at the same voltage.

The charge (material being treated) never solidifies in the anode-crucible 12, since it must be poured in a liquid state out of the furnace. As already explained, the liquid material is poured out of the furnace, for example by tilting the furnace, or by manual extraction, or using pumping means, such as pneumatic pumping means, among others. For this reason (because the material to be treated must be extracted in a liquid state, and therefore, the operating temperature must be kept high enough throughout the process), the crucible 12 and its surrounding layers (refractory layers and metal structure) are not refrigerated, to prevent the material to be treated from solidifying inside the crucible 12. For this reason, it is necessary to design optimized refractory layers. If the crucible must be cooled, a much greater amount of energy would be required to maintain the treated material in a molten state until it is removed from the furnace.

Returning to the anode system, Figures 4B-4D show different views of the first elongated portion 12'. The second 12'' elongated portion is not shown. It can be implemented with a cylinder, either solid or hollow, having, for example, a thread at the end that will connect to the first elongated portion 12'. In other words, the two elongated parts 12' 12'' connect the bottom of the crucible 12 with the outside of the tank 10, allowing electrical connection between the crucible 12 acting as an anode and the DC power source. The parts 12', 12" are, therefore, part, together with the crucible 12, of the anode system. Figures 5A-5B show the three parts 12, 12', 12'' that form the anode system, once assembled .

In the embodiment of Figures 4A-4D, the lower portion 12B of the crucible 12 has a minimum thickness of 120 mm (millimeters, 10.3 meters) and a maximum thickness of 250 mm. The side wall 12A (perimeter wall) of the crucible 12 has a minimum thickness of 25 mm and a maximum thickness of 75 mm. By way of example, when a furnace has a crucible of 530 mm internal diameter and the cathode is configured to provide a plasma jet of about 160 kW (kilowatts, 103 watts), the bottom 12B of the crucible 12 can be selected to which is 160 mm and the side wall 12A (perimeter wall) of the crucible 12 can be selected to have a thickness of 35 mm.

The anode system 12, 12', 12'' and the cathode 13 are easily replaceable. In particular, the 12' 12" parts allow for easy replacement of the anode. Since the anode system is made in this embodiment of three independent removable parts, and since the material surrounding them (between the graphite parts of the anode and the refractory walls, such as concrete walls) is preferably a self-sintering refractory, if necessary, the graphite parts are removable and the self-sintering material is easy to remove. Therefore, the three main refractory concrete bodies are maintained. No damage and removed parts can be easily replaced.

As explained, the anode (crucible 12) is preferably made of a material that comprises at least 98% synthetic graphite (percentage expressed by weight with respect to the total weight of the material) and an amount of ash content equal to or less than 2 % (percentage expressed by weight with respect to the total weight of the material). In a particular example, the anode (crucible 12) is made of a material comprising at least 98% synthetic graphite obtained from a material comprising more than 30% acicular coke and graphitized at a temperature of at least 2800 °C. The apparent density of the material is at least 1.6 g/cm3, preferably between 1.65 and 1.75 g/cm3. The selected synthetic graphite provides high conductivity properties both within the melting temperature range, typically 600 to 1600°C, and at room temperature, such as less than 50°C. In both temperature ranges, electrically short conditions (such as, for example, < 10 volts between anode and cathode) must be obtained to allow current to pass through the electrical circuit. Therefore, the maximum allowable resistivity is 5.8 O x pm measured in the longitudinal direction of the current passage.

The thermal conductivity of the material from which the anode is made is also a key property for easy homogenization of the thermal conditions within the chamber 11 formed by the anode 12. The thermal conductivity of the material from which the anode is made must be higher at 125 W/Km, preferably greater than 200 W/km.

Since the values of the expansion and contraction process of the different materials that make up the oven are not the same, the anode 12 must be able to withstand expansion and contraction forces. Therefore, the material that constitutes the anode must have a flexural strength of at least 9 MN/m2 and a Young's modulus preferably between 8 and 10 MPa. Its linear resistance must be at least 4.5 MPa.

Figure 2 shows a possible implementation of the layers, walls or portions 14A, 14B, 14C, 14D, 14E of refractory materials arranged between the crucible 12 and the metal frame 15 that constitutes the outer surface of the furnace 1. The refractory layers 14A- 14E are intended to prevent leaks, such as metal leaks, and provide high insulation. In other words, the main body of the tank 10 is made of at least one refractory material adapted to the characteristics of the material to be melted/treated.

Non-limiting examples of refractory materials that can be used are concrete or brick. In the implementation of Figure 2, different refractory materials are used that have different properties and functions.

The layer 14A, which vertically surrounds the perimeter wall of the crucible 12, may be made of alumina-based corundum. In a particular embodiment, the layer 14a is made of alumina-based self-sinterable corundum having an amount of alumina (aluminum oxide, AhOa) greater than >85%, an amount of magnesia (magnesium oxide, MgO) between 10 and 15%, an amount of iron (III) oxide (Fe2Oa) less than 0.3% and an amount of silica (silicon dioxide, SiO2) less than 0.2% (percentages expressed by weight with respect to the total weight of the compound or composition). The grain size (largest dimension) is less than 6 mm and the apparent density is between 2.6 and 2.9 g/cm3. This material has a maximum working temperature of 1800 °C. The layer 14A preferably extends downward beyond the inner base of the crucible 12, but without extending beyond the entire thickness of the bottom of the crucible 12 (on average to the average thickness of the bottom 12B of the crucible 12). . The thickness of layer 14A may be between 5 and 30 mm.

A second portion or layer 14B of refractory material is arranged vertically surrounding the first layer 14A. This layer 14B may be made of alumina-based concrete. In a particular embodiment, the layer 14B is made of formed alumina-based concrete having an amount of alumina greater than 90% (preferably 94-97%) with contents of SiO2 < 0.5% and Fe2O3 < 0.5% (percentages expressed by weight with respect to the total weight of the compound or composition). The thickness of layer 14B ranges between 25-100 mm. The maximum working temperature of this material is greater than 1800 °C, or greater than 1850 °C.

A third portion or layer 14C of refractory material is arranged below the lower portion of the crucible 12. Preferably it extends radially, such that it is also arranged below the first and second layers 14A 14B, as well as below the first elongated portion 12' of the anode system. This layer 14B comprises calcined bauxite. It is capable of withstanding a maximum temperature of 1,700 °C. In a particular embodiment, the layer 14C is made of a mixture of calcined bauxite in an amount between 80-90% and alumina-based concrete in an amount between 10-20% (percentages expressed by weight with respect to the total weight of the compound or composition). The thickness of the 14C layer can be variable. For example, its thickness may be greater under the first and second layers 14A, 14B than under the bottom of the crucible 12. The average thickness of the layer 14C at the bottom of the crucible is between 50 and 100 mm. A small layer 14C' of the same material as layer 14C surrounds the upper end of part 12''.

A fourth portion or layer 14D of refractory material is arranged vertically surrounding the second layer 14B. This layer 14D is the last refractory wall before the metal frame 15. The layer 14D preferably extends downward with respect to the third layer 14C, in such a way that it surrounds all the previous refractory portions arranged around the crucible 12. This layer 14B comprises also calcined bauxite. It is capable of withstanding a maximum temperature of 1,500 °C. In a particular embodiment, the layer 14D is made of a mixture of calcined bauxite in an amount between 77-85% and alumina-based concrete in an amount between 15-23% (percentages expressed by weight with respect to the total weight of the composite or composition). The average thickness of this layer is between 50-250 mm.

Surrounding the second elongated portion 12'' of the anode system extending vertically from the first elongated portion 12' thereof, is a layer or portion 14E of refractory material. It can be a self-sintering refractory based on silica. Since the maximum temperature in this portion 12' is lower than the temperature in the refractory walls closest to the crucible, a material that has a lower sintering temperature can be used, such as a silica-based self-sintering refractory.

The space (if any) between the outer frame of the furnace and the detailed refractory layers can be filled with insulating fiber and low-density bauxite insulating refractories. These materials can also be used to fill the 14F layer at the bottom of the oven. All these layers or portions 14A-14F provide high insulation capacity.

By controlling the furnace atmosphere, the free oxygen content within chamber 11 is minimized, for example reduced to less than 1%. The furnace atmosphere is controlled (low oxygen content) by purging inert gas inserted through the cathode and adding reducing material (carbon base) with the material to be heated. In chamber 11 of furnace 1, the reactions of the reduction-oxidation process can be controlled by applying precise temperature control and ensuring specific conditions of the interior atmosphere. The furnace can be used in the reduction of metal oxides contained in minerals, by-products and industrial waste.

Below, several experiments related to metallurgical reactions are disclosed, carried out with a furnace implemented according to the figures shown.

In a first experiment, tin oxides from non-ferrous detinning processes are reduced in an oven as shown in Figure 1. The electrode diameter of the plasma torch is 75 mm (+/-3%). A maximum electrical power of 160 kW and an average power of 100-110 kWh are used for a treatment of 200 kg/h of material. The average composition to be treated is: 70-85% SnO/SnO2; 5-20% Cu/CuO; Zn < 5%; Fe < 2%; Ag < 0.5%; Ni < 1%; others including for example SiO2 or AhO3 < 8%. It has been treated in the oven at an average temperature between 1,050 °C and 1,180 °C, with a minimum process temperature of 950 °C and a maximum process temperature of 1,250 °C. The atmosphere is reduced to A< 1 (where the value of A is CO2max/CO2 = 1 (O2/(21-O2)). Slag conditioning elements containing different amounts of SiO2, Al2O3, Na3(AIF6), MgO and Fe2O3 are added and mixed with the tin oxide raw material. In the chamber 11 defined by the crucible 12, the following reactions are carried out:

xSnO yC = xSn CO2

SnO C = SnCO

CuO C = CuCO

SnO2 2CO = 2Sn 2CO2

Zn(s)(+Q) = Zn(g)

While the smoke is captured and supplied to the extraction duct 25, a reoxidation process occurs (from Zn to ZnO, in this example). Reoxidation occurs as the material melts inside the furnace. The fumes are extracted from the oven as already explained.

Zn(g)O = ZnO(g)

As a result of the reduction or refining process, the molten metal bath is obtained with the following compositions: Sn: 85-95%; Cu: 5-15%; Fe: <0.2%; Ni: < 0.2%; Zn < 0.5%. Additionally, slag and filter dust are obtained as by-products.

The average composition of the slag is CuO<5%, SnO 10-25%, Fe2O3 5-15%, AI2O3 5-25% MgO <5%, SiO2 5 20% CaO 3-8% and variable amounts of C (< 5% and Na2O<25%. Rest of elements <3%.

The composition of the filter dust is SnO 50-75%, ZnO 10-25%, Na2O 5-20% and other residual elements <3%.

In the same oven used in the first experiment, a second experiment is carried out. It is a refining process by reducing copper oxides from the cutting and stripping of non-ferrous copper-based scrap, with an average composition of: Cu < 40%; CuO 60-90%; Sn < 1%; Ni < 1%; Fe <2%; Zn < 5%; others (including for example SiO2, Al2O3, MgO,...) < 10%. It has been treated in the oven at an average temperature between 1,150 °C and 1,300 °C, with a minimum process temperature of 1,050 °C and a maximum process temperature of 1,350 °C. The atmosphere is reduced as A< 1 (where the value of A is CO2max/CO2 = 1 (O2/(21-O2)). Slag conditioning elements containing different amounts of SiO2, Al2O3, Na3(AIF6), MgO and Fe2O3 are added and mixed with the tin oxide raw material. In the chamber 11 defined by the crucible 12, the following reactions are carried out:

CuO C = CuCO

xSnO yC = xSn CO2

SnO C = SnCO

SnO2 2CO = 2Sn 2CO2

Zn(s)(+Q) = Zn(g)

And then, in the smoke capture system:

Zn(g)O = ZnO(g)

As a result of the reduction or refining process, a molten metal bath is obtained with the following compositions: Cu: > 98%; Sn Fe Ni: < 2%; Zn < 0.5%.

In this text, the term "comprises" and its derivatives (such as "understanding", etc.) should not be understood in an exclusive sense, that is, these terms should not be interpreted as excluding the possibility that what is described and defines may include other elements, stages, etc.

In the context of the present invention, the term "approximately" and its family terms (such as "approximate", etc.) should be understood as indicating values very close to those accompanying the aforementioned term. That is, a deviation within reasonable limits from an exact value must be accepted, because a person skilled in the art will understand that such a deviation from the indicated values is inevitable due to measurement inaccuracies, etc. The same applies to the expressions "approximately" and "about" and "substantially".

On the other hand, the invention is obviously not limited to the specific embodiment(s) described herein, but also encompasses any variations that may be contemplated by those skilled in the art (for example, regarding the choice of materials , dimensions, components, configuration, etc.), within the general scope of the invention as defined in the claims.

Claims (13)

1. A direct current plasma arc furnace (1) for melting and/or treating a material by producing electric arcs and providing a molten product, the furnace (1) comprising: a tank (10) comprising: a crucible (12) that delimits a chamber (11) configured to receive material to be melted and/or treated; a plurality of refractory walls surrounding the outer surface of the crucible (12); and a metal frame (15) that covers the refractory walls; a heating system configured to heat the received material, the heating system comprising a first electrode that acts as a cathode (13) and a second electrode (120) that acts as an anode, wherein the first electrode that acts as a cathode (13) It is a mobile electrode configured to project vertically into the chamber (11) delimited by the crucible (12); wherein the crucible (12) is made of a material comprising at least 98% by weight of synthetic graphite, the oven (1) being characterized in that the crucible (12) is part of an anodic system, the anodic system formed by the crucible (12), connecting said second electrode (120) and at least one part (12', 12'' ) the crucible (12) and the second electrode (120), said crucible (12) having a double function: receiving and containing material to be melted and/or treated and providing electrical conduction for the flow of current to heat said material, in such a way so that, when the furnace is in use (1), the voltage potential difference between the cathode (13) and any point on the surface of the crucible (12) defined to be in contact with the material to be melted and/or treated it's the same; and wherein said at least one part (12', 12") of the anode system comprises: a first elongated portion (12') having a first end connected to the lower wall of the crucible (12) and extending radially from the same; and a second elongated portion (12'') having a first end connected to the second end of the first elongated portion (12'), the second elongated portion (12'') extending vertically until its second end (120) appears out of the oven (1). 2. The oven (1) of claim 1, wherein the heating system comprises a plasma torch. 3. The furnace (1) of any one of claims 1-2, wherein the cathode (13) is made of a material comprising at least 98% by weight of synthetic graphite. 4. The oven (1) of any one of claims 1-3, wherein the plurality of refractory walls arranged between the outer surface of the crucible (12) and the metal frame (15) comprises: a first layer (14A) of refractory material that vertically surrounds the perimeter wall of the crucible (12); a second layer (14B) of refractory material that vertically surrounds the first layer (14A); a third layer (14C) of refractory material arranged under the bottom of the crucible (12); and a fourth layer (14D) of refractory material that vertically surrounds the second layer (14B) and in contact with the outer metal frame (15). 5. The furnace (1) of claim 4, wherein the first layer (14A) comprises alumina-based corundum. 6. The furnace (1) of claim 5, wherein the first layer (14A) is made of self-sintering alumina-based corundum having an amount of alumina greater than 85% by weight. 7. The oven (1) of any of claims 4-6, wherein the second layer (14B) comprises alumina-based concrete. 8. The furnace (1) of claim 7, wherein the second layer (14B) is made of formed alumina-based concrete having an amount of alumina greater than 90% by weight. 9. The furnace (1) of any of claims 4-8, wherein the third layer (14C) and the fourth layer (14D) comprise calcined bauxite. 10. The furnace (1) of claim 9, wherein the third layer (14C) and the fourth layer (14D) also comprise alumina-based concrete. 11. The furnace (1) of any of claims 4-10, wherein the fourth layer (14D) extends downward with respect to the third layer (14C), surrounding the previous refractory portions arranged around the crucible (12). . 12. The oven (1) of any one of claims 4-11, further comprising a fifth layer (14E) of refractory material surrounding the second elongated portion (12") of the anode system, the fifth layer (14E) being a self-sintering silica-based refractory. 13. Use of the oven (1) of any preceding claim to melt or treat metals and/or metal waste and/or byproducts containing metals and other chemical compounds.